Method for crucible free zone melting



Sept. 6, 1966 W. KELLER METHOD FOR CRUCIBLE FREE ZONE MELTING FiledSept. 8, 1964 United States Patent 3,271,551 METHQD FOR CRUCIBLE FREEZONE MELTING Wolfgang Keller, Pretzfeld, Germany, assignor toSiemens-Schuckertwerke Aktiengesellschaft, lierliniemensstadt, Germany,a corporation of Germany Filed Sept. 8, 1964, Ser. No. 394,834 Claimspriority, application Germany, Sept. 6, 1963, S 87,138 1 Claim. (Cl.219-1041) My invention relates to a method of crucible free zone meltingof a rod-shaped body held at its ends, in which a melting zone isproduce-d by means of an induction heater surrounding the body.

In crucible free zone melting of semiconductor material or the like, amelting zone is usually produced in a rod-shaped semiconductor body thatis supported at its ends. The melting zone is passed through the bodydue to relative motion between the semiconductor rod and the heatingdevice. It is known to use as heating device an induction coilsurrounding the rod and connected parallel with a capacitor in a heatingcircuit which is coupled to an output circuit of a high-frequencygenerator having a frequency of 3 to 5 rnI-Iz., for example.

It is customary to have the high frequency generator operate at such afrequency that, when the rod zone is molten, the operating point lies onthe inductive portion of the voltage-current characteristic, this beingthe curve portion of the current-voltage characteristic of the heatingcircuit which rises with an increase in conductivity (German Patent962,006).

In crucible free zone melting with inductive heating of the meltingzone, it is also known to use the current supplied to the heatingcircuit as a measure of the coupling degree and hence as indicative ofthe thickness of the rodshaped body being processed, thus employing thevariation in heating current for regulating the thickness of the body.Preferably, the distance between the two rod holders is controlled independence upon these current variations so as to move the rod holderstoward one another when the inductive coupling decreases, thuscompressing and thickening the melting zone between the two rod portionsand thereby increasing the degree of coupling. This affords automaticregulation of the rod diameter, for example by comparing the currentdrawn by the heating circuit with a datum value, and moving one of therod holders upon deviation from this datum value in the direction and bythe amount required to eliminate the deviation.

Such a diameter regulation of the rod-shaped body has also beenperformed by tuning the high-frequency generator and the heating circuitso that the high-frequency generator operates on the inductive side ofthe resonance curve of the heating circuit. However, this involvesdifficulties with relatively thick rods, for example silicon rods ofover 20 mm. diameter, causing the regulating mechanism to get out ofstep occasionally. With such large rod diameters the regulation may passbeyond the resonance point of the resonance curve; and the regulatingdevice will then operate in the wrong direction, which results infailure of the regulating performance.

It is therefore an object of my invention to overcome the difiicultiesof the known methods and particularly to prevent the regulatingmechanism from getting out of step when working with relatively thicksemiconductor rods.

According to my invention, I provide a method for crucible free zonemelting in which the semiconductor rod is inductively heated by a coilenergized from a high-frequency generator, which always operates on thecapacitive side of the resonance curve of the heating circuit whenmelting the rod at the melting zone.

Other features which are considered as characteristic for my inventionare set forth in the appended claim.

Although the invention has been illustrated and described as a methodfor crucible free zone melting, it is nevertheless not intended to belimited to the details shown since various modifications may be madetherein without departing from the spirit of the invention and withinthe scope and range of equivalents of the claim.

The method of this invention, however, together with additional objectsand advantages thereof will be best understood from the followingdescription when read in connection with the accompanying drawings, inwhich:

FIG. 1 is a partly schematic view of an apparatus employed for carryingout the method of my invention;

FIG. 2 is a graph showing the resonance curve of the heating circuit;and

FIG. 3 is an enlarged view of the melting zone in the rod-shaped body ofthe apparatus shown in FIG. 1.

The zone melting is carried out preferably within a hell or otherreceptacle under high vacuum, and the semiconductor rod and its holdersas well as the inductance heating coil and, if desired, also the devicesfor displacing the holder, are mounted within the receptacle. Suchdesign features which are not essential to an understanding of theinvention proper herein and which are known per se for apparatus of thisgeneral type are not illustrated in the drawing.

Referring now to the drawings and particularly to FIG. 1, there is showna rod-shaped body 2 of semiconductor material such as silicon, germaniumor the like held between two holders 3 and 4. A heating coil 5 surroundsthe rod and produces a melting zone 6 by means of inductive heating. Theheating coil 5 is so arranged that it can be moved along the rod 2 at agiven speed, and is connected in parallel with capacitor 7 to form aninductive heating or tank circuit therewith. The heating coil 5 andcapacitor 7 are connected to the output terminals 8 of a high-frequencygenerator 9 which is energized for example, by a direct current source10. The high-frequency generator 9 is of the type shown and described inPatent No. 3,046,379 of which I am a coinventor. For the purpose ofcontrol or regulation, the plate current I (see FIG. 2) of thehigh-frequency generator 9 which is proportional to the current fed intothe heating circuit is conducted through a variable resistance 11serially connected in the plate circuit of the generator 9. The voltagedrop developed across the resistance 11 is measured with a suitablyconnected voltmeter 12a. The voltage drop across the resistor 11 acts onthe coil of a relay 13 in a differential relation to an adjustable datumvoltage from a source 12.

The relay 13 is polarized, and its contacts 14 are in an inactiveposition when the voltage drop across resistor 11 is substantially equalto the datum voltage of source 12. However, the contacts 14 close eitherone or the other of the two stationary contacts respectively, which arereversely connected to the poles of a current source 16, when thevoltage drop across resistor 11 and hence the magnitude of the generatorcurrent deviates from the datum value i.e. increases or decreasesrespectively.

The relay 13 thus operates the reversing contacts 14, to energize amotor 15 connected to the direct-current source 16. Depending upon theincrease or decrease of the heating circuit current and the consequentcorresponding deviation in the voltage drop across the resistor 11 ascompared to the datum voltage of the source 12, the contacts 14 willengage one or the other of the stationary contacts respectively toeffect rotation of the motor 15 in one or the other rotary directionrespectively. The motor 15, for example, drives a gear 18 through areduction gear system (not shown) in a desired rotary direction so thatthe toothed rack 17 with which it is in meshing engagement and which isconnected with the displaceable holder 3 is displaced respectivelytoward or away from the melting zone 6. The holder 4 is fixed againstdisplacement in the direction of the rod axis but is however, rotatableabout the rod axis by means such as is disclosed in the aforementionedPatent No. 3,046,379, which also discloses suitable means for displacingthe heating coil along the rod.

The high-frequency generator 9 generally operates on a side of theresonance curve of the induction heating or tank circuit formed by thecoil 5 and capacitor 7. A typical resonance curve is shown in FIG. 2with voltage values U applied to the heating circuit or plate current Iof the high-frequency generator as ordinate and the frequency values jas abscissa, both at a linear rate. If the high-frequency generator 9operates on the inductive or ascending side, for example at point I, ofthe resonance curve of the heating circuit 5, 7, as shown in FIG. 2, thefollowing regulation takes place upon deviation of the rod diameter froma desired value. A change takes place in the coupling of the meltingzone 6 to the heating coil 5 when there is a deviation of the roddiameter from the desired value, and the current supplied to the heatingcircuit 5, 7 and thereby also the plate current of the high-frequencygenerator varies accordingly. This causes variation in the voltage dropacross the resistance 11, so that the relay 13, suitably polarized,actuates the relay plunger to move in the necessary direction to switchthe reversible motor 15 with suitable polar connections to the currentsource 16 so that the motor rotates the gear 18 for displacing the rack17 and the holder 3 in the appropriate direction. The deviation from thedesired rod diameter value is thus again equalized or adjusted 5 and thesemiconductor material crystallizing from the melting zone shows areduced deviation from the desired value, or relatively no deviation atall.

It has been found that when using a so-called fiat coil as for example,the one depicted in FIG. 3, particularly, the melting of the rod doesnot take place continuously but rather, in an intermittent manner. Whenassuming for example, that the flat coil 5a in FIG. 3 is travellingupwardly, then, the fixed rod parts 2a and 2b, the melting zone 6a andthe heating coil 511 will have substantially the relative spatialarrangement shown in FIG. 3. The rod material thus melts at the upperpart of the melting zone 6a and crystallizes again at the lower partthereof. At times, an isolated portion or island of the rod part 2awhich has slowly begun to glow suddenly becomes molten and flowsdownward into the bulge of the melting zone 6a. This abruptly loosensthe coupling between the heating coil 5a and the melting zone 6a, sincethe heating coil is also located in the upper region of the meltingzone, i.e. at its narrow neck. This neck is suddenly constricted, due tothe sudden melting of a portion of the solid semiconductor material and,hence, loosens heating circuit, which as previously mentioned, can causethe operating point to pass over and beyond the resonance point or peakof the resonance curve. Consequently, the regulation falls out of stepand, as a rule, causes the melting zone to harden and interrupts thezone melting operation.

If, in accordance with my invention, the high-frequency generator andthe heating circuit are so tuned or matched that the high-frequencygenerator operates on the capacitive side of the resonance curve of theheating circuit, for example at point II in FIG. 2, this dangerous swingor aberration of the control devicecan then no longer occur. Naturallyin this instance, the control means have to be adjusted to the oppositepolarity than if the generator were operated on the inductive side ofthe curve. Preferably, the control means should be so constructed thatregulation or control therewith can take place along the inductive aswell as the capacitive side, i.e. at the operating points I and II. Insuch a case the melting zone can pass upwardly along the semiconductorrod when the control means is operating on the capacitative side of theresonance curve whereas, during the downward passage of an incandescentzone through the semiconductor rod, the control means operates on theinductive side.

By employing auxiliary devices for varying the field of the inductiveheating coil, such as levitating coils or short circuit rings, thenormal form of the melting zone is disturbed. It is, thereforeadvantageous to carry out my inventive method Without using suchauxiliary devices and only by using a flat coil such as 'is illustratedin FIG. 3.

I claim:

In a method of zone melting a semiconductor rod in which the rod isvertically supported at both ends and a molten zone is formed in the rodby a surrounding inductive heating coil connected in parallel with acapacitor and forming therewith a heating circuit having specificresonance curve characteristics, the heating circuit is energized bycurrent from a high-frequency generator varying in accordance with achange in the diameter of the rod for actuating a regulating device tovary the axial spacing between the rod ends until the current assumes adatum value, and the zone is caused to move lengthwise of the rod, theimprovement which comprises operating the high-frequency generator at afrequency in the capacitive leg of the resonant curve of the heatingcircuit for melting the rod at the zone while moving the zone upwardlyalong the rod, and operating the high-frequency generator at a frequencyin the inductive leg of the resonant curve of the heating circuit formaintaining the zone as an incandescent solid while moving the zonedownwardly along the rod.

References Cited by the Examiner UNITED STATES PATENTS 2,913,561 11/1959Rummel et a1 219--10.43

FOREIGN 'PATENTS 962,006 4/ 1957 Germany.

RICHARD M. WOOD, Primary Examiner.

ANTHONY BARTIS, Examiner.

L. H. BENDER, Assistant Examiner.

